Sulfite oxidases from mammalian and avian sources have been shown to consist of domain structures. Both molybdenum and heme containing domains have been isolated from rat liver sulfite oxidase while a similar heme containing domain has been isolated and characterized from chicken liver sulfite oxidase. Additionally, sequence homology has been shown to exist among the chicken liver sulfite oxidase heme domain and the heme domains of microsomal cytochrome b5 and yeast lactate dehydrogenase. The major emphasis of this project is to obtain much needed information on the effects of evolutionary pressures on protein structure. This problem will be approached by purifying and characterizing sulfite oxidase from a bacterial source (Thiobacillus novellus) and comparing its domain, structural, and immunochemical characteristics with those of the sulfite oxidase from mammalian and avian sources. The same enzyme present in organisms widely separated in evolutionary time is likely to have structural and enzymic properties that are unique to each organism. The experiments proposed in this project are designed to discover the unique differences between evolutionarily distinct enzyme molecules, and to evaluate the effects of these differences on enzymic activity. It is believed that the experiments proposed here will result in a clear understanding of how a given biochemical reaction may assume unique properties characteristic of a particular organism.